MIT builds a mini-magnetar to look for dark matter particles

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If there’s anything scientists and the public seem to agree on, it’s how baffling the hunt has been for dark matter. Since we just can’t seem to find dark matter using our current methods, what’s next? How do we put some data behind this hypothesis? Or at least put it to bed in favor of something we can substantiate?

I don’t know about you, but if I hear “dark matter is a placeholder” one more time I’m going to start yelling “the mitochondria is the powerhouse of the cell” until someone puts me out of my misery.

Evidently some people from MIT feel the same, because a trio of their wunderkinder are taking a different tack in dark-matter research. They plan to use MRI magnet-wrangling tech to build a palm-sized model magnetar, the most powerful magnetic object in the universe, in their lab. The mini-magnetar rig is called ABRACADABRA, for A Broadband/Resonant Approach to Cosmic Axion Detection with an Amplifying B-field Ring Apparatus. Thaler, physics fellow Benjamin Safdi, and postdoc partner Yonatan Kahn plan to use their device to look for a hypothetical particle called an axion, which is capable of interacting with magnetic fields like the one they’re building. It could help turn up evidence of dark matter. It could even contribute to solving the charge-parity problem in particle physics.

The idea is they’ll wrap a magnetometer in a series of magnetic coils, wrap that in a sheet of superconductor, and stuff the whole thing inside a purpose-built cryogenic refrigerator, like a turducken of particle physics. Then they’ll plug it in and watch its magnetic field. In the presence of axions, the magnetic field created by the turducken should fluctuate, just a little bit. The transit of the axion through the mini-magnetar’s B-field causes a tiny subordinate magnetic field to pop into being. The researchers are betting they can pick up that tiny pop and wobble, since they know to great precision what their own mini-magnetar’s B-field should look like. If they find it, they can use it to estimate the axion’s size.

So let’s have a bit of real talk about why anyone cares about an axion at all, and how we expect to find them, if they can be found.

If the original Peccei-Quinn theory of their niche in the cosmos holds true, axions would saturate space, a little like neutrinos: tiny, light, and everywhere. This whole thing spirals quickly away into a heady whirl of vectors and double integrals if you let it. But it boils down to this: there is a fine-tuning problem in our model of subatomic particles. To wit, the weak nuclear force doesn’t obey the expected balance of charge parity, which is weird because reasons. Quantum reasons. It’s called the charge-parity problem, or the Strong CP problem.

Nor do neutrons behave as theory predicts they should: “We don’t expect neutrons to accelerate in the presence of an electric field because they don’t carry electric charge, but you might expect them to rotate,” says Safdi. That’s because we expect them to have an electric dipole moment, since the Pauli exclusion principle dictates that the neutron’s constituent charges have to be separated in space and thus off-center. But neutrons don’t appear to have the expected dipole, which is also weird.

“The Strong CP problem is associated with whether a neutron’s spin responds to electric effects, and you can kind of think of a magnetar as one gigantic spin with big magnetic fields,” explains Thaler in the MIT writeup. Axions detected by ABRACADABRA (“detected by ABRACADABRA”? Do you guys even hear yourselves saying these words at your lab meetings?) could be the common answer. Particles can also be understood through the dynamic of fields, and finding these missing interactions would crack both the charge parity and neutron spin problems wide open.

These experiments also have implications for the dark matter question. While you’ve probably already heard of WIMPs as a candidate for “the dark matter particle,” researchers have been talking quietly about axions since the late 1970s. The University of Washington is basically the only place doing axion research — so “axion” isn’t exactly a household name. But it’s in a class of particles that have a lot in common with WIMPs. Like a WIMP, the axion is supposed to be feebly-interacting and ultralight, predicted to have around a quadrillionth or quintillionth the mass of a proton: again, a great candidate for explaining something we haven’t been able to explain yet.

“We have an instrument that’s sensitive to many wavelengths, and we can tickle it with an axion of one particular wavelength, and ABRACADABRA will resonate,” says Thaler. If this mini-magnetar does find axions, and if their mass is within the expected range, their relevancy to research will rapidly climb.

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